Abstract
Several surface modified ultrafine silica powders have been prepared by a dehydration-condensation reaction between a hydroxy (silanol) group on the surface of silica and a normal-alkyl chain group of different alcohols. By comparing the adsorption isotherms of water molecules on the differently modified silica surfaces, the one-to-one correspondence in water adsorption was found between water molecules and residual surface silanol groups within the range of relative water vapor pressure (P/P0) in the formation of monomolecular adsorption layer on the silica surface. It was found that such a long chain of alkyl groups as normal-octyl or dodecyl groups can make the silica surface hydrophobic preventing from capillary condensation at an interparticle space, even at a small degree of alkyl group substitution (0.3-0.4 groups/nm2) due to a great increase of an effective molecular occupation area of substituted alkyl chain groups. It was evidentially pointed out that the bound water of at least two to four molecular layers adsorbed on the silica surface exists in an energetically more stable state due to structurization of water molecules in the layers than normal water being depressed in its freezing point.
